Thermo sensor

[Tatt00z]

Good morning, I recently finished my repower of 2 1998 OX66 250 motors. Everything was basically plug and play except for one sensor on top of both motors. Just a light blue and a black wire coming out of it. I can't seem to find anything online about it or more importantly where it plugs in. The seller had everything marked pretty good, but when I got to that it just plugged into a black and blue wire at the instrument cluster, but the leads were taped up and didn't go anywhere. Any and all information is greatly appreciated. Also only one alarm sounds when port motor starts nothing on starboard. Ignition starts motors fine, but I believe I need both alarms. Since the reason for the repower was a blown motor, I believe I want all my warnings working properly.
Thanks Glen

 

[DennisG01]

Yamaha service manuals are relatively cheap and can be an invaluable tool to have. Highly recommended. It would definitely answer your question in just a few minutes of paging through. I have one that stays on the boat (500 miles away from me) and another at home... although looked for it last night for another reason and couldn't find it. I'll have to ask my kids about that...

But, and I'm only going from memory here, it sounds like that wire/sensor you're talking about could be the O2 sensor?

 

[seasick]

Good morning, I recently finished my repower of 2 1998 OX66 250 motors. Everything was basically plug and play except for one sensor on top of both motors. Just a light blue and a black wire coming out of it. I can't seem to find anything online about it or more importantly where it plugs in. The seller had everything marked pretty good, but when I got to that it just plugged into a black and blue wire at the instrument cluster, but the leads were taped up and didn't go anywhere. Any and all information is greatly appreciated. Also only one alarm sounds when port motor starts nothing on starboard. Ignition starts motors fine, but I believe I need both alarms. Since the reason for the repower was a blown motor, I believe I want all my warnings working properly.
Thanks Glen

There are two temperature sensors and one thermo switch. There could be a water temp sensor or water pressure sensor. The first three devices are pressed into a rubber bushing on the head(s) and the later two (pressure and water temp) would be threaded into the head or block.
On the smaller SX motors, the colors you mentioned do not match the dock for thermo switch or temp sensor. I would think that the colors would be the same for the larger SXs

All of the first three sensors connect to the ECU and not the instrument cluster directly. Other sensors may have been run to specific gauges like water temp or pressure.

There are ways to test the alarm. A low oil level in the on engine oil tank will do that . Remove the sensor from the tank, make sure that the float ring is all the way down and then turn on the ignition (you no not have to start the motor). The alarm should sound.

 

[Tatt00z]

Thanks, that's some great information. While googling O2 sensors I came a crossed a pic of it, not the one the arrow is pointing at, but the one directly behind it with the rusted connectors. Thanks again Glen

 

[seasick]

Thanks, that's some great information. While googling O2 sensors I came a crossed a pic of it, not the one the arrow is pointing at, but the one directly behind it with the rusted connectors. Thanks again Glen

I believe the O2 is the one that the arrow is pointing to.
The one with the pink and black wires is the thermo sensor.
I am not sure if the one in the middle is the temp sensor. What colors are its wires?

 

[Tatt00z]

Blue and black. Thanks

 

[seasick]

I was wrong about one thing:
There are two thermo sensors, one for each bank. They switch on high temp.
There is one temp sensor, its output (resistance) changes with temperature and is used by the ECU to adjust fuel mix, cold idle etc.
There is a knock sensor. It should have one wire connected to it or so it seams.
I have not yet found what sensor has blue and black wires but this diagram might help

 

[seasick]

 

[Tatt00z]

Thanks for all your help, I'm going down to a friend's boat this week and see if I can find where it plugs in, he has same motors but 2001,hopefully they will be the same

 

[Tatt00z]

According to this diagram, it looks like it would be the thermo switch. A Blue and a white wire, abd I still have no idea where it plugs in.
Thank you , you have been a great help.

 

[DennisG01]

As luck would have it, I happen to have a spare OX66 250HP in my garage, a '97. I also found the service manual... it was on my workbench. No comments, please!

According to my engine and service manual, anyways, the thermoswitch (the one with the rust connector) has 2 wires - BOTH are Black w/Yellow stripe. The thermosensors (one on each bank) also have two wires - each has a black one and a pink one. I don't see ANY two wire connectors up there with a light blue wire and a black wire.

BUT, although it's a strange thing to chance wire color coding, if we assume something changed from my engine to yours, then where mine plugs in is along the aft side of the large plastic box. It also follows the green wires from the crank position sensor. The thermoswitch connector plugs in below the connector for the CPS's wires (the thermoswitch connector is tied together with the lower CPS green wires)... at the bottom, aft corner of the big plastic box.

The only Sky Blue (light blue) wires I see are for the power trim and tilt (up). But that would also have a Light Green wire and either a red or black wire, depending on which end you're at. But a 3-wire connector, regardless.

 

[seasick]

As luck would have it, I happen to have a spare OX66 250HP in my garage, a '97. I also found the service manual... it was on my workbench. No comments, please!

According to my engine and service manual, anyways, the thermoswitch (the one with the rust connector) has 2 wires - BOTH are Black w/Yellow stripe. The thermosensors (one on each bank) also have two wires - each has a black one and a pink one. I don't see ANY two wire connectors up there with a light blue wire and a black wire.

BUT, although it's a strange thing to chance wire color coding, if we assume something changed from my engine to yours, then where mine plugs in is along the aft side of the large plastic box. It also follows the green wires from the crank position sensor. The thermoswitch connector plugs in below the connector for the CPS's wires (the thermoswitch connector is tied together with the lower CPS green wires)... at the bottom, aft corner of the big plastic box.

The only Sky Blue (light blue) wires I see are for the power trim and tilt (up). But that would also have a Light Green wire and either a red or black wire, depending on which end you're at. But a 3-wire connector, regardless.

If the thermo sensor were not connected the ECU would have an issue. At a minimum, the motor would not idle correctly when cold or would not idle down when it warms. ( I am nor sure how it handles an open circuit)

The thing I find most interesting is that Dennis just so happens to have a 250 in his garage! Dang.. Had I known that earlier I wouldn't have spent so much time trying to find the wire colors using the service manual harness tiny diagrams

 

[DennisG01]

The thing I find most interesting is that Dennis just so happens to have a 250 in his garage! Dang.. Had I known that earlier I wouldn't have spent so much time trying to find the wire colors using the service manual harness tiny diagrams

That's alright, I did both! I looked at the engine AND looked at those tiny, tiny diagrams!

Yeah... it is kinda nice to have a full size, spare parts bin! After I bought the boat, the owner said "Oh, by the way, I have an extra engine if you want it". It has one scored cylinder, which is why the guy replaced it (original engine is the '97 in my garage, the boat has a '00). Came in handy, too - the lighting coil crapped out on the boat engine. Took the one from the '97 and good to go!

 

[Tatt00z]

I'll ride down tomorrow and take a few pics and post them up. Thanks guys for all your help

 

[Tatt00z]

Ok gentlemen, here goes a pic of the alarm/sensor I am trying to figure out. Thanks for everything.

 

[DennisG01]

I don't have that sensor on my spare engine. It is not listed in the service manual for my spare engine. Nor is it listed in any online parts diagrams for 1998 or 1999. HOWEVER, I DO have a bolt/plug in that spot (on both sides of the engine). The bolt/plug is also noted in my service manual and the online diagrams.

You said the wires go all the way to the dash? How did you confirm that? Ohm meter? I'm just going to take a guess that it was an add-on temperature sensor that went to a stand-alone gauge at the dash, at some point during it's previous owners.

For the alarm issue... I have never done it on my Yamaha, but on Mercruisers I will sometimes swap things like the alarm buzzers to see if the problem follows the buzzer or follows the engine. With the Yamaha gauges, is the buzzer built into the gauge? In that case, swap the gauges. Double check me on all that - although I can't imagine why that wouldn't work?

Oh... whether it's a stand-alone buzzer, or built in, there should be 12V going to it with the key on. Use the working side to verify, and then check the non-working side. If you have 12V and the buzzer doesn't sound, check the ground wire - even jumping a wire to ground. If the "grounding" works, then I "think" there is an issue from that grounding wire back to the computer. Again, please double check me, but I "think" the grounding is controlled by the computer?

 

[seasick]

I don't have that sensor on my spare engine. It is not listed in the service manual for my spare engine. Nor is it listed in any online parts diagrams for 1998 or 1999. HOWEVER, I DO have a bolt/plug in that spot (on both sides of the engine). The bolt/plug is also noted in my service manual and the online diagrams.

You said the wires go all the way to the dash? How did you confirm that? Ohm meter? I'm just going to take a guess that it was an add-on temperature sensor that went to a stand-alone gauge at the dash, at some point during it's previous owners.

For the alarm issue... I have never done it on my Yamaha, but on Mercruisers I will sometimes swap things like the alarm buzzers to see if the problem follows the buzzer or follows the engine. With the Yamaha gauges, is the buzzer built into the gauge? In that case, swap the gauges. Double check me on all that - although I can't imagine why that wouldn't work?

Oh... whether it's a stand-alone buzzer, or built in, there should be 12V going to it with the key on. Use the working side to verify, and then check the non-working side. If you have 12V and the buzzer doesn't sound, check the ground wire - even jumping a wire to ground. If the "grounding" works, then I "think" there is an issue from that grounding wire back to the computer. Again, please double check me, but I "think" the grounding is controlled by the computer?

The buzzer is in the ignition key switch assembly.
I also could not find a sensor with those colors. As Dennis mentioned it is probably a separate temp sensor or perhaps a water pressure sensor. If either, having it disconnected will not do any harm.

 

[Tatt00z]

Thanks guys for all your help, the reason I said it went all the way back to the gauges is thats what the previous owner had told me, but they just ran nowhere when they got to the cluster. Makes sense that it is an add on , and done by the owner before the guy who sold them to me. I swapped the harnesses hoping the ignition alarm would change sides, I suspect one of the harnesses are bad since I need to"jiggle" it sometimes for it to operate properly. Anyway sometime this winter I will purchase a tach and harness for that engine, and possibly a new ignition.
Thanks again.